Recessed lighting fixtures are disposed within a hole or trough cut into a wall or suspended ceiling. In particular, a “can” housing a light bulb is disposed within the hole or trough and the wiring is above the ceiling or behind the wall. The can has trim that covers the hole if the hole's diameter is larger than the can's.
Fires often spread through a building by passing through the light fixture or a portion of the hole and then moving through the space above the ceiling or behind the wall. The recessed light fixture itself provides little resistance to a spreading fire because it is often made from materials that conduct heat and the trim does not adequately seal the opening in the ceiling.
There are structures that attempt to prevent the fire from passing into the space above the ceiling. Such structures include a box usually made of a relatively heavy and bulky, thermally insulating material, such as compressed mineral wool or plaster board sheet rock